Problem: On the first 5 tests of his geometry class, Luis got an average score of 88. What does he need on the next test to have an overall average of 90?
Let his score on the next test be $x$ The sum of all of his scores is then $5 \cdot 88 + x$ The same sum must also be equal to $6 \cdot 90$ Solve: $x = 6 \cdot 90 - 5 \cdot 88 = 100$.